Measuring and testing – Moisture content or absorption characteristic of material
Reexamination Certificate
2001-10-11
2003-09-09
Williams, Hezron (Department: 2856)
Measuring and testing
Moisture content or absorption characteristic of material
C073S03200R, C073S437000, C702S137000
Reexamination Certificate
active
06615643
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to methods and systems used to determine the specific gravity, absorption, and/or porosity characteristics of compacted and loose materials including aggregate materials used in the construction of roads and structures as well as those obtained in connection with oil and geological explorations.
BACKGROUND OF THE INVENTION
Water absorption and specific gravity of aggregates are both parameters which are routinely analyzed in the design and construction of roads and structures worldwide. These parameters can also be important considerations in oil and geological explorations.
The ability to accurately measure water absorption and specific gravity of materials in a repeatable manner and in a relatively short time frame can be important for engineers and practitioners interested in assessing the suitability of bulk materials and material mixtures in their projects. For example, water absorption and specific gravity values can yield important information about the hydraulic properties of soils and aggregates.
In the asphalt mix design industry, the bulk specific gravity and absorption of aggregates in a particular design, which can include both fine and coarse aggregates, are important assessments of the quality and suitability of the asphalt design to a particular application. The design selection of materials can be a mixture or composition of various sized aggregates in an assortment of different materials which can be varied to yield the desired functional characteristics or standards. Bulk specific gravity can be used as a measure to assess the amount of asphalt binder absorbed by the aggregates and the percentage of voids in the mineral aggregates in the design; each of these parameters can be important considerations in assessing the quality of the materials or the suitability of the composition of the design.
Conventionally, test methods described in standards AASHTO T84 and ASTM C128 have been used to assess fine aggregates. Unfortunately, these methods can have poor repeatability. Generally stated, the conventional method requires that a material sample of fine aggregate (about 1000 g) is oven dried to a constant weight. The material sample is then immersed in water for a 24-hour saturation period. The sample is then spread on a flat surface and exposed to a gently moving stream of warm air until a saturated surface-dry condition is reached. To assess when the saturated surface-dry condition has been reached, the material sample is positioned into an inverted cone and lightly compacted. The cone is removed and if the material “slumps” the material sample is considered to be in a saturated surface-dry condition. The amount of “slump” that represents when the saturated surface-dry condition has been reached can vary from test-to-test and is operator-dependent. Some laboratories or agencies define this condition as one in which the slump corresponds to the diameter of a dime from the top of the cone. The amount of slump can be adjusted by repetitive drying of the aggregates until the desired slump is achieved. However, if the aggregate sample is over-dried during the test procedure, the sample must be re-saturated and the drying process repeated.
After the material sample has reached the saturated dry-surface condition, a portion of the material sample is placed in a flask, which is then filled with water to a calibrated level and weighed. The fine aggregate material sample is removed from the flask and oven-dried to a constant weight. The specific gravity (apparent and bulk) and absorption are then calculated based on the three measured weights (the weight of the oven-dried sample, the weight of the flask filled with water, and the weight of the flask with the material and specimen and water to a calibration mark).
Angular fine aggregates with high absorption characteristics and/or rough surface textures do not typically slump readily. Therefore, determining the saturated surface dry (SSD) weight for samples that include these types of materials can be difficult with the cone method described above. Unfortunately, incorrect determination of this parameter in the testing process can have undesirable effects on the performance or service life of the asphalt pavement or other structure made using incorrectly analyzed materials.
In the concrete industry, the same cone test is typically used to determine the SSD condition in fine materials to determine the proper amount of water to add to the concrete mixture. Proportioning the concrete mixture with an incorrect amount of water can negatively affect the strength and durability of the concrete structures.
The testing standards for coarse aggregates are described in AASHTO T85 and ASTM C-127. “Coarse” is typically associated with aggregates retained on a 2.36 mm (No. 8) or larger sieve. In order to obtain the SSD weight of these types of samples, these standards provide that the operator pads the aggregates with a towel and uses the towel-dried weight as the SSD weight of the sample. Again, this technique is subjected to operator variability, as if the material sample is not properly prepared—such as if improper washing or wetting of the sample, aggressive drying, or removing fine dirt particles off the surfaces of the aggregates (thus, potentially leaving the large aggregate surface wet)—the results of the analysis can vary and may not provide a reliable indication of the properties of the sample. Further, the towel-dry technique itself is a subjective procedure and the degree of dryness can vary from operator-to-operator and sample-to-sample.
Recently, a study was undertaken by the National Center for Asphalt Technology (NCAT) and was presented at the 79
th
meeting of the Transportation Research Board, in January, 2000. In this study, the authors proposed a device to attempt to automate the determination of the SSD condition for fine aggregates as a replacement to AASHTO T84 and ASTM C128. The device included a spinning drum equipped with a hair dryer for drying the aggregates, a humidity indicator and a temperature sensor mounted inside the drum. In operation, a saturated material sample is placed inside the drum and the sample is spun while continuously monitoring temperature and humidity. The theory behind this technique is that a break in the response between temperature and time or humidity and time will indicate a saturation point. For example, continuous drying will occur until either the temperature or humidity stabilizes. At this “stability point”, the aggregates are expected to be at the SSD condition. After the indicated response has stabilized, the temperature or humidity can continue to change, also indicating that the internal water has been removed (another indication that the SSD condition was achieved at the stability point). Unfortunately, in operation, the material can clump together inside the drum. When aggregates clump (fine aggregates can be particularly susceptible to clumping), the SSD condition may be unachievable. Indeed, fine aggregates can impede accurate determination of a true SSD condition as they have a tendency to stack up or attach to each other and not allow the surface of each individual aggregate to reach the desired SSD condition. Further, the stability point (defined as a plateau) in time versus temperature or humidity is an empirical derivation that may be difficult to ascertain or achieve with every aggregate type.
Recently, another device has been proposed by the Barnstead/Thermolyne Company of Boise, Idaho to determine the SSD condition of fine aggregates. This device proposes placing approximately 500 g of dry aggregates in a vibrating dish. Water is introduced into the aggregate and an infrared device monitors the surface moisture. Again, the time response versus the infrared moisture reading is plotted and a point along the response line is identified and selected as corresponding to the SSD condition of the aggregates. Unfortunately, this method is also empirically based and can depend on the type and perhaps the gradation of aggr
James Lawrence
Regimand Ali
Garber Chares D
InstroTek, Inc.
Myers Bigel & Sibley Sajovec, PA
Williams Hezron
LandOfFree
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